1. Field of the Invention
This invention is in the field of random access multiplexers for multiplexing signals from photovoltaic (PV) detectors from a large array of such detectors located in the focal plane of a photosensing device, and more particularly, relates to multiplexer elements, or cells, of a multiplexer each of which is capable of integrating signals from a photovoltaic detector, of providing a low noise readout, and of providing random access to the integrated signals of each photovoltaic detector of a large array of such detectors.
2. Description of the Prior Art
The usual prior art approach to multiplexing signals from photovoltaic detectors of a large array of detectors onto one or more output lines is to collect charge packets at an input site of the multiplexer, there being one input site for each detector. The magnitude of the charge collected in a given period of time at each site is representative of, or a function of, the detector signal. There are two basic ways of reading out the signals accumulated at each input site. One is by using charge integrating devices (CIDs) and a second is by using charge coupled devices (CCDs). In a CID, the integrated charge packet is transferred to a sense electrode, the voltage of which is proportional to the input charge divided by the capacitance of the sense electrode. CIDs can provide random access readout, however, they are limited to use with smaller arrays of detectors because the capacitance of the sense electrode, or node, increases as the number of detectors of the array increases, with the result that the output signal voltage decreases because of the increase in capacitance of the sense electrodes. As a result, the signal to noise ratio rapidly decreases to unacceptable levels as the number of detectors of the array increases.
CCDs transfer the input charge packet from an input well to an output node by transferring the charge packet for each detector of the array from one potential well to the next in a series of such wells constituting the CCD. Use of CCDs is limited by the dynamic range of CCDs, by noise, and by fixed pattern artifacts introduced as a charge packet is transferred from one potential well to the next.
A third approach to multiplexing signals from a large number of voltaic detectors is to use a switched array which sequentially switches each detector to a common output line. Such an approach allows random access to each photovoltaic detector's output signal, but has a high background noise as the result of the non-integrating, non-band width limiting characteristics of the signal transfer process.